This deformations appeared in this 3D round model that I'm trying to create
You are probably using the "Subdivision modifier" applied to the car mesh.
You should learn how to control this modifier (it's one of the most complex things in 3d software). Here some basic info: https://docs.blender.org/manual/en/latest/modeling/modifiers/generate/subdivision_surface.html#control

The easiest way to go on with your work is to disable the subdivision modifier and work in a "low poly / flat / shade flat" way. 

Once you mastered basic modeling, you can try experimenting also the "subdivision" things (here one of many tutorials you'll find on youtube: https://www.youtube.com/watch?v=xmIIIcCUm8E)